Chemical bonding method and joined structure

ABSTRACT

A bonded structure includes a first substrate; a second substrate placed opposite to the first substrate; an intermediate layer provided between the first substrate and the second substrate and including a first oxide thin film layered on the first substrate and a second oxide thin film layered on the second substrate; either or both of the first oxide thin film and the second oxide thin film of the intermediate layer being formed of oxide thin films having increased defects; and an interface between the first oxide thin film and the second oxide thin film=being bonded by chemical bonding, and the interface comprising a low-density portion whose density is lower than that of the two oxide thin films.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.17/592,881, filed on Feb. 4, 2022, which is a continuation ofPCT/JP2020/033471, filed on Sep. 3, 2020, and which claims the priorityof JP 2019-162065, filed on Sep. 5, 2019, and JP 2020-098031, filed onJun. 4, 2020. The subject matter of U.S. application Ser. No.17/592,881; PCT/JP2020/033471; JP 2019-162065; and JP 2020-098031 isincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a chemical bonding method and a bondedstructure having a bonded portion bonded by the chemical bonding method,and more specifically to an improved atomic diffusion bonding method inwhich a thin film of metal or semi-metal formed in a vacuum vessel onthe bonding surface of one of the substrates to be bonded issuperimposed on a thin film of metal or semi-metal formed on the othersubstrate or on the bonding surface of the other substrate for bonding,and to a bonded structure having a bonded portion bonded by thisimproved chemical bonding method.

BACKGROUND OF THE INVENTION

Bonding techniques that bond two or more materials to be bonded are usedin various fields, and such bonding techniques are used for, forexample, wafer bonding and package sealing in the field of electroniccomponents.

Taking the above-described wafer bonding technique as an example, therelated-art general wafer bonding technique is to apply high pressureand high heat between superimposed wafers to bond them.

However, this method of bonding cannot be used to bond or integratesubstrates provided with electronic devices that are sensitive to heator pressure. Therefore, there is a need for a technique to bond thesubstrates to be bonded without physical damage such as heat orpressure.

As one of such bonding techniques, a bonding method called “atomicdiffusion bonding” has been proposed.

In this atomic diffusion bonding, a microcrystalline thin film(hereinafter referred to as the “bonding film”) of metal or semi-metalwith a thickness of the nano-order is formed on a smooth surface of oneof the wafers, chips, substrates, packages, or other various materials(hereinafter referred to as “substrate”) to be bonded by vacuumdeposition such as sputtering or ion plating, and the thin film issuperimposed on a bonding film formed on a smooth surface of othersubstrate or on a smooth surface of a substrate with a microcrystallinestructure in the same vacuum as that used to form the bonding film, orunder atmospheric pressure, thereby allowing bonding with atomicdiffusion at the bonding interface and grain boundaries (see PatentDocuments 1 and 2).

This atomic diffusion bonding can be used to bond any materials to bebonded that allow the formation of the above-described bonding film in avacuum. Therefore, not only semiconductor and ceramics wafers, but alsometals, ceramics blocks (substrates), polymers, and various othermaterials can be bonded, and in addition to bonding similar materials,different materials can be bonded without heating, preferably at roomtemperature (or low temperature).

Atoms of a metal in the solid phase can hardly move at room temperature.However, in atomic diffusion bonding, the large surface energy of thebonding film formed in a vacuum vessel is used as the driving force forbonding, and bonding is performed by transferring the atoms of thematerials constituting the bonding film at room temperature using thelarge atomic diffusion performance on the surface of the bonding filmand the crystal lattice rearrangement at the contact interface.

The atomic diffusion and crystal lattice rearrangement on the surfaceare low-energy, high-speed movements of atomic defects (vacancies) onthe surface and at the bonding interface, which can be used to movemetal atoms and bond them at room temperature.

Atomic diffusion bonding can be performed at room temperature using abonding film of any metal. In particular, materials with larger atomicself-diffusion coefficients, such as Ti and Au, allow for easier atomicmovement and rearrangement of crystal lattice at the bonding interface,resulting in higher bonding performance.

Among these atomic diffusion bonding methods, bonding in a vacuum usinga thin metal film of about a few angstroms (a few angstroms per sidewhen bonding films together) as the bonding film on the bonding surfaceproduces a bonding interface that is transparent and has almost noelectrical conductivity, and is therefore being considered for use inbonding optical components and new functional devices.

However, even if the bonding film at the bonding interface is a thinmetal film of only a few angstroms on each side, the thin film hascharacteristics similar to those of metal, which means that about 1 to2% of light is absorbed at the bonding interface and a small amount ofelectrical conductivity remains, and this small amount of remaininglight absorption and conductivity can be a problem in the formation ofhigh brightness devices and electronic devices.

In order to solve this problem, a method for producing an elastic wavedevice has been proposed (Patent Document 3), in which the metal bondingfilm existing at the bonding interface is oxidized after bonding byatomic diffusion bonding to make it lose conductivity, therebypreventing characteristic degradation due to leakage of high frequencysignals into the metal bonding film.

Specifically, Patent Document 3 discloses a method for producing anelastic wave device by laminating a thin film of piezoelectric materialand a support substrate; first, an oxide base layer is formed on bothbonding surfaces, and then a metal bonding film is formed on top of theoxide base layer, and then the bonding films are superimposed on eachother for atomic diffusion bonding. Thereafter, heat treatment isperformed and the bonding films are oxidized by the oxygen that deviatesfrom the oxide base layer during the heat treatment to form a metaloxide film, thereby eliminating the degradation of the characteristicsof the elastic wave device caused by the presence of metal bonding filmsat the bonding interface (see paragraphs [0012] to [0018], [0028] ofPatent Document 3).

RELATED ART DOCUMENTS Patent Document

-   [Patent Document 1] Japanese Patent No. 5401661-   [Patent Document 2] Japanese Patent No. 5569964-   [Patent Document 3] Japanese Patent KOKAI No. 2015-222970 (LOPI;    automatically published after around 18 months from filing date    regardless prosecution)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the bonding method of the piezoelectric material and the supportsubstrate used in the method for producing an elastic wave devicedescribed in the above-described Patent Document 3, the metal bondingfilm existing at the bonding interface is oxidized by heat treatmentafter atomic diffusion bonding, and the characteristic degradationcaused by the presence of the metal bonding film at the bondinginterface can be favorably eliminated.

However, this bonding method not only requires thickness control to forma very thin metal bonding film of a few A, but also requires a newprocess to form an oxidation base film, heat treatment for oxidationafter atomic diffusion bonding, and control to stably supply the bondingfilm with the amount of oxygen that deviates from the oxidation basefilm during the heat treatment. As a result, there are many parametersthat need to be controlled for mass production, and these becomeobstacles in increasing productivity.

The reason for the formation of an oxide base layer, the addition of anew heat treatment process after atomic diffusion bonding, and the needfor complex parameter management during the heat treatment and others isto oxidize the metal bonding film that exists at the bonding interfaceafter atomic diffusion bonding. If atomic diffusion bonding can beperformed using a bonding film formed by oxides from the beginning,there is no need to oxidize the bonding film after the bonding, whicheliminates the need for the process of forming the oxidized base layer,the heat treatment for oxidation, and the complicated work of managingthe parameters during the heat treatment.

Thus, in order to perform atomic diffusion bonding using the oxide thinfilm as the bonding film, atoms must move around on the surface of theoxide thin film and cause chemical bonding with the surface of the otheroxide thin film (or substrate).

However, the atomic diffusion bonding described in the above-describedPatent Documents 1 and 2 is based on the premise that the bonding filmformed on the bonding surface is formed of metal or semi-metal. Theseliteratures not only suggest that it is not possible to form a bondingfilm with an oxide or to perform atomic diffusion bonding with a oxidethin film, but also neither disclose nor suggest that atomic diffusionbonding is possible with a bonding film formed with such an oxide.

That is, in atomic diffusion bonding, metal atoms are transferred andbonded at room temperature by utilizing the large atomic diffusionperformance on the surface of the bonding film and the crystal latticerearrangement at the contact interface, as described above. Therefore,the bonding film formed on the bonding surface must be one that easilycauses atomic diffusion, and it was thought that the higher the surfacediffusion coefficient, which indicates the ease of atomic diffusion, theeasier it is for atoms to be bonded, and the lower the surface diffusioncoefficient, the less likely atoms are to be bonded at room temperature.

Therefore, the above-described Patent Documents 1 and 2 specify thebonding conditions according to the surface diffusion coefficient Ds ofa metal or semi-metal that is the material of the bonding film, anddescribe that it is desirable to increase the surface diffusioncoefficient by heating when bonding using a metal or semi-metal bondingfilm with a low surface diffusion coefficient Ds at room temperature(claim 4 of Patent Document 1), and that the heating temperatureconditions during bonding should be increased along with the decrease inthe surface diffusion coefficient (paragraphs [0070] to [0073] of PatentDocument 2).

Thus, Patent Document 1 and Patent Document 2 suggest that even whenbonding is performed using a bonding film composed of a non-oxidizedmetal or semi-metal, bonding may not be possible at room temperaturewhen the bonding film is formed with a material with a small surfacediffusion coefficient.

In contrast to the surface diffusion coefficients Ds of metals andsemi-metals, the surface diffusion coefficients of these oxides areremarkably low. As an example, as shown in Table 1 below, for iron (Fe),the surface diffusion coefficient Ds of atoms on the individual surfaceof its oxide, iron oxide (Fe₂O₃), is much lower than that of either theγ-structure, which is a high-temperature phase, or the α-structure,which is stable near room temperature, and is 11 orders of magnitudesmaller than that of the γ-structure and 13 orders of magnitude smallerthan that of the α-structure.

TABLE 1 Surface diffusion coefficient Ds Element (cm²/s) Temperature (°C.) Literature* Fe₂O₃ oxide 10⁻⁸ 900 to 1100 (1) Fe (γ structure) 4 ×10³ 920 to 1100 (2) Fe (α structure) 10⁵   740 to 880  (2) *Literature(1): Yasuro Ikuma, Wazo Komatsu, “Surface Diffusion and Surface Layersin Ceramic Materials,” Journal of the Surface Science Society of Japan,Vol. 5, No. 1, pp. 12-21 (1984). *Literature (2): Kenichi Hirano, RyojiTanaka, “Surface Diffusion of Metals”, Bulletin of the Japan Instituteof Metals and Materials, Vol. 9, No. 6, pp. 341-358 (1970)

Table 1 above shows the surface diffusion coefficients Ds at hightemperatures that can be evaluated experimentally. Even at roomtemperature, the surface diffusion coefficient Ds of iron oxide (Fe₂O₃)is still an order of magnitude smaller than that of iron (Fe), and thisrelationship applies not only to iron and iron oxide, but also to othermetals and semi-metals and their oxides.

Therefore, it is the understanding of the person skilled in the art,including the inventors of the present invention, that even if bondingfilms composed of a metal or semi-metal oxide are superimposed on eachother, no atomic movement occurs, and therefore, atomic diffusionbonding cannot be performed. Based on this understanding, as introducedin Patent Document 3 above, development has been carried out on thepremise that, in atomic diffusion bonding, the only way to form abonding film at the bonding interface with an oxide is to oxidize thebonding film after the bonding is completed.

However, as a result of intensive research by the inventors of thepresent invention, it was found that, contrary to the recognition of theperson skilled in the art as described above, under certain conditions,an unexpected result could be obtained in which chemical bonding couldbe performed even when the bonding film was initially formed with anoxide.

The present invention is based on the above findings obtained as aresult of research by the inventors, and aims to provide a chemicalbonding method that enables bonding by means of an oxide thin film,thereby obtaining a bonded structure in which the bonding interface isbonded through an oxide thin film without the formation of an oxide baselayer or heat treatment for oxidation after bonding, and withoutcomplicated parameter management associated with heat treatment foroxidation.

Means for Solving the Problem

In order to achieve the object, a chemical bonding method of the presentinvention comprises; in a vacuum vessel, forming amorphous oxide thinfilms having increased defects and facilitating movement of atoms onsmooth surfaces of two substrates each having a smooth surface, andsuperimposing the two substrates so that the amorphous oxide thin filmsformed on the two substrates are in contact with each other, therebycausing chemical bonding at a bonding interface between the amorphousoxide thin films to bond the two substrates.

Immediately after film formation, fine crystals may be partially formedinside the amorphous oxide thin film. However, such partially existingfine crystals do not interfere with chemical bonding, and a thin filmthat partially contains such fine crystals is included in the amorphousoxide thin film in the present invention.

Furthermore, a chemical bonding method of the present inventioncomprises; in a vacuum vessel, forming an amorphous oxide thin filmhaving increased defects and facilitating movement of atoms on a smoothsurface of one substrate, and superimposing two substrates so that theamorphous oxide thin film formed on the one substrate contacts a smoothsurface of other substrate having a smooth surface with an oxide thinfilm at least on its surface, thereby causing chemical bonding at abonding interface between the amorphous oxide thin film and the smoothsurface of the other substrate to bond the two substrates.

Moreover, a chemical bonding method of the present invention comprises;in a vacuum vessel, forming an amorphous oxide thin film havingincreased defects and facilitating movement of atoms on a smooth surfaceof one substrate, and superimposing two substrates so that the amorphousoxide thin film formed on the one substrate contacts a smooth surface ofother substrate having an activated smooth surface, thereby causingchemical bonding at a bonding interface between the amorphous oxide thinfilm and the smooth surface of the other substrate to bond the twosubstrates.

In any of the above chemical bonding methods, preferably, the chemicalbonding involves atomic diffusion at the bonding interface.

Furthermore, in any of the above chemical bonding methods, preferably,arithmetic mean roughness of the surface of the amorphous oxide thinfilm is 0.5 nm or less.

Preferably, the amorphous oxide thin film may be formed by a methodinvolving rapid cooling of raw material atoms on the smooth surface ofthe substrate, furthermore, preferably, bonding is performed by formingthe amorphous oxide thin film having increased defects on the smoothsurface of the substrate. In such a defective amorphous oxide thin film,the state of chemical bonding on the surface is not stabilized and thereare many broken chemical bonds, which can easily cause chemical bondingwith the surface of the other amorphous oxide thin film (or theactivated substrate).

The superimposing of the substrates may be performed without heating thesubstrates, or the substrates may be heated when the substrates aresuperimposed at a temperature from room temperature to 400° C. toaccelerate the chemical bonding.

Preferably, forming the amorphous oxide thin film and superimposing thesubstrates is performed in a vacuum vessel with an ultimate vacuumpressure of from 1×10⁻³ Pa to 1×10⁻⁸ Pa. Furthermore, formation of theamorphous oxide thin films and the superposition of the substrates arepreferably performed in the same vacuum.

Preferably, a difference in electronegativity between oxygen and one ormore of elements other than oxygen constituting the amorphous oxide thinfilm is 1.4 or more, or ionicity defined in the following equation is40% or more:

Ionicity (%)=[1−exp{−0.25(B−A)²}]×100

wherein A is the electronegativity of one or more of the elements otherthan oxygen constituting the amorphous oxide thin film, and B is theelectronegativity of oxygen.

The amorphous oxide thin film may comprise one or more elements selectedfrom the element group of Be, Mg, Al, Sc, Ti, V, Cr, Mn, Zn, Y, Zr, Nb,Hf, Ta, Li, Na, K, Ca, Rh, Sr, Cs, Ba, Fe, Co, Ni, Cu, Ag, Ge, Ga, In,Sn, B, Si.

Preferably, a thickness of the amorphous oxide thin film is from 0.3 nmto 5 μm, more preferably, from 0.5 nm to 1 μm.

A bonded structure of the present invention comprises:

a first substrate;

a second substrate placed opposite to the first substrate;

an intermediate layer provided between the first substrate and thesecond substrate and including a first oxide thin film layered on thefirst substrate and a second oxide thin film layered on the secondsubstrate;

either or both of the first oxide thin film and the second oxide thinfilm of the intermediate layer being formed of oxide thin films havingincreased defects;

an interface between the first oxide thin film and the second oxide thinfilm being bonded by chemical bonding, and the interface comprising alow-density portion whose density is lower than that of the two oxidethin films.

Another bonded structure of the present invention comprises:

a first substrate;

a second substrate placed opposite to the first substrate;

an intermediate layer provided between the first substrate and thesecond substrate and including an oxide thin film having increaseddefects layered on the first substrate;

an interface between the oxide thin film of the intermediate layer andthe second substrate being bonded by chemical bonding, and the oxidethin film at the bonded portion having a low-density portion whosedensity is lower than that of the oxide thin film.

Furthermore, preferably, the interface between the first oxide thin filmand the second oxide thin film of the intermediate layer is bonded bychemical bonding with atomic diffusion.

The interface between the oxide thin film of the intermediate layer andthe second substrate may be bonded by chemical bonding with atomicdiffusion.

In the bonded structures, a material constituting the oxide thin film ofthe intermediate layer may be different from a material constituting thefirst substrate or the second substrate.

Effect of the Invention

According to the configuration of the present invention as describedabove, in the chemical bonding method of the present invention, thebonding film formed on a smooth surface of a substrate is an amorphousoxide thin film formed in a vacuum vessel, which enables chemicalbonding to be performed without heating even with an oxide thin filmthat was thought to be incapable of chemical bonding due to its lowsurface diffusion coefficient.

As a result, a bonded structure with an oxide thin film formed at thebonding interface was obtained without the need for an oxide base layer,heat treatment after bonding, or complicated parameter management duringheat treatment.

Thus, the thin film at the bonding interface can be an oxide thin film,and the bonding interface can be formed by an oxide thin film withoutlight absorption or conductivity, thereby expanding the range ofapplication of atomic diffusion bonding by chemical bonding to highbrightness devices, optical devices, electronic devices, and otherdevices where light absorption and conductivity at the bonding interfacecause functional degradation.

It has already been mentioned that in order for chemical bonding tooccur by an amorphous oxide thin film, it is necessary for atoms to moveeasily on the surface of the contacting oxide thin film and for chemicalbonding such as ionic bonding and covalent bonding to occur with thesurface of the other oxide thin film (or substrate).

Whether it is an amorphous structure or a crystalline structure, themost stable composition for an oxide thin film is the one determinedstoichiometrically by the valence of oxygen and the elements such asmetals that form the oxide, and in this stable composition, atoms do notmove easily, and therefore, chemical bonding is difficult to occur.

However, in an amorphous oxide thin film formed by rapid cooling ofhigh-temperature raw material atoms sputtered from a target surface bysputtering or the like on a substrate surface, a thin film havingincreased defects, such as a state in which oxygen is missing oroversaturated compared to the above-described stoichiometriccomposition.

In such a thin film having increased defects, there is a largefluctuation in the bonding state between the ions of elements such asmetals and oxygen, which facilitates the movement of atoms on thesurface of the thin film, thus making chemical bonding easier to occur.Therefore, even though an oxide thin film is used, atoms are transferredat the bonding interface and stable chemical bonding is obtained, whichis thought to be the reason why the two substrates can be bonded firmly.

In particular, when atomic diffusion occurred at the bonding interface,the bonding strength could be improved by stabilizing the chemicalbonding over a wider range.

In oxides, since the binding energy between the metal or semi-metalelement and oxygen is large, the diffused atoms are immediately trappedby nearby heteroatoms, and the range in which they can move is veryshort, making it difficult for them to bond with heteroatoms at distantlocations.

Therefore, when the surface of the amorphous oxide thin film is roughand there is even a small gap at the bonding interface, the bondingstrength will decrease even if bonding is possible because chemicalbonding does not occur in this area.

However, in the configuration where the arithmetic mean roughness of thesurface of the above-described amorphous oxide thin film was 0.5 nm orless, it was possible to make contact with the entire area of the filmsurface at the atomic level during bonding, resulting in strong chemicalbonding.

In the chemical bonding method of the present invention, bonding can beperformed even when no heat treatment is performed after bonding,however by performing heat treatment at 400° C. or lower after bonding,the chemical bonding at the bonding interface is further enhanced,resulting in even stronger bonding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the correlation between thickness and surface roughness(arithmetic mean height Sa) of an amorphous oxide thin film (TiO₂ thinfilm).

FIG. 2 shows the correlation between the thickness of an amorphous oxidethin film (TiO₂ thin film) and the bonding strength (surface free energyof the bonding interface) γ.

FIG. 3 shows a cross-sectional electron micrograph (TEM) of a Siwafer-Si wafer bonded using a TiO₂ thin film as an amorphous oxide thinfilm (bonded and heat-treated at 300° C. for 5 minutes).

FIG. 4 shows a cross-sectional electron micrograph (TEM) of a Siwafer-Si wafer (unheated) bonded using an Y₂O₃—ZrO₂ thin film as anamorphous oxide thin film.

FIG. 5 shows a cross-sectional electron micrograph (TEM) of a Siwafer-Si wafer bonded using an Y₂O₃—ZrO₂ thin film as an amorphous oxidethin film (after bonding and heat treatment at 300° C. for 5 minutes).

FIG. 6 shows a cross-sectional electron micrograph (TEM) of a Siwafer-Si wafer (bonded and unheated) bonded using an Y₂O₃ thin film asan amorphous oxide thin film.

FIG. 7 shows the correlation between the change in the bonding strength(surface free energy of the bonding interface) γ with respect to thechange in the thickness for quartz substrate—quartz substrate bondedusing an Nb₂O₅ thin film.

FIG. 8 shows the correlation between the change in surface roughness Sawith respect to the change in the thickness of an Nb₂O₅ thin film formedon a quartz substrate.

FIG. 9 shows a cross-sectional electron micrograph (TEM) of a Siwafer-Si wafer bonded using an Nb₂O₅ thin film as an amorphous oxidethin film (bonded and heat-treated at 300° C. for 5 minutes).

FIG. 10 shows the correlation between the change in the bonding strength(surface free energy of the bonding interface) γ with respect to thechange in the thickness for quartz substrate—quartz substrate bondedusing an Al₂O₃ thin film.

FIG. 11 shows the correlation between the change in surface roughness Saand the change in thickness of an Al₂O₃ thin film formed on the quartzsubstrate.

FIG. 12 shows a cross-sectional electron micrograph (TEM) of a Siwafer-Si wafer bonded using an Al₂O₃ thin film as an amorphous oxidethin film (bonded and heat-treated at 300° C. for 5 minutes).

FIG. 13 shows the correlation between the change in the bonding strength(surface free energy of the bonding interface) γ with respect to thechange in the thickness for quartz substrate-quartz substrate bondedusing an ITO thin film.

FIG. 14 shows the correlation between the change in surface roughness Sawith respect to the change in the thickness of an ITO thin film formedon a quartz substrate.

FIG. 15 shows a cross-sectional electron micrograph (TEM) of a Siwafer-Si wafer bonded using an ITO thin film as an amorphous oxide thinfilm (bonded and heat-treated at 300° C. for 5 minutes).

FIG. 16 shows the correlation between the change in the bonding strength(surface free energy of the bonding interface) γ with respect to thechange in the thickness for quartz substrate—quartz substrate bondedusing a Ga₂O₃ thin film.

FIG. 17 shows the correlation between the change in surface roughness Saand the change in thickness of a Ga₂O₃ thin film formed on a quartzsubstrate.

FIG. 18 shows a cross-sectional electron micrograph (TEM) of a Siwafer-Si wafer bonded using a Ga₂O₃ thin film as an amorphous oxide thinfilm (bonded and heat-treated at 300° C. for 5 minutes).

FIG. 19 shows the correlation between the change in the bonding strength(surface free energy of the bonding interface) γ with respect to thechange in the thickness for a quartz substrate-quartz substrate bondedusing a GeO₂ thin film.

FIG. 20 shows a cross-sectional electron micrograph (TEM) of a Siwafer-Si wafer bonded using a GaO₂ thin film as an amorphous oxide thinfilm (after bonding, heat-treated at 300° C. for 5 minutes).

FIG. 21A shows the correlation between the electronegativity of theoxide-forming elements and the bonding strength (surface free energy atthe bonding interface) γ of amorphous oxide thin films (both films are 2nm thick) in the case where the films are unheated after bonding.

FIG. 21B shows the correlation between the electronegativity of theoxide-forming elements and the bonding strength (surface free energy atthe bonding interface) γ of amorphous oxide thin films (both films are 2nm thick) in the case where the films are heat-treated at 300° C. for 5min after bonding.

FIG. 22A shows the correlation between the ionicity (%) of theoxide-forming elements and the bonding strength (surface free energy atthe bonding interface) γ of amorphous oxide thin films (both films are 2nm thick) in the case where the films are unheated after bonding.

FIG. 22B shows the correlation between the ionicity (%) of theoxide-forming elements and the bonding strength (surface free energy atthe bonding interface) γ of amorphous oxide thin films (both films are 2nm thick) in the case where the films are heat-treated at 300° C. for 5min after bonding.

FIG. 23 illustrates the “blade method” used to measure the bondingstrength (surface free energy of the bonding interface) γ.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The chemical bonding method of the present invention is described below.

[Overview of Bonding Method]

The chemical bonding method of the present invention performs chemicalbonding using an amorphous oxide thin film formed by vacuum filmdeposition such as sputtering or ion plating in a vacuum vessel,including superimposing the amorphous oxide thin films formed on smoothsurfaces of two substrates to be bonded, or

superimposing the amorphous oxide thin film (first oxide thin film)formed on the smooth surface of one of the substrates to be bonded onthe smooth surface with an oxide thin film (second oxide thin film)formed on the other substrate, furthermore,

superimposing the amorphous oxide thin film formed on the smooth surfaceof one of the substrates to be bonded on the smooth surface of the othersubstrate having an activated smooth surface to generate chemicalbonding, preferably chemical bonding with atomic diffusion at thebonding interface to bond the two substrates.

[Substrate (Material to be Bonded)]

(1) Material

The substrate to be bonded by the chemical bonding method of the presentinvention may be any material on which the above-described amorphousoxide thin film can be formed by sputtering, ion plating, etc., in ahigh vacuum atmosphere using a vacuum vessel with an ultimate vacuum offrom 1×10⁻³ to 1×10⁻⁸ Pa, preferably from 1×10⁻⁴ to 1×10⁻⁸ Pa, as anexample. In addition to various pure metals and alloys, semiconductorssuch as Si wafers and SiO₂ substrates, glass, ceramics, resins, oxides,etc. that can be vacuum deposited using the above method may be used asthe substrate (material to be bonded) in the present invention.

The substrate can be bonded not only between the same materials, such asmetal to metal, but also between different materials, such as metal andceramics.

(2) State and Other Properties of Bonding Surface

The shape of the substrate is not particularly limited, and may be anyshape from a flat plate to various complex three-dimensional shapes,depending on the application and purpose. However, the part to be bondedwith the other substrate (bonding surface) must have a smooth surfaceformed with a predetermined accuracy.

This smooth surface, which is bonded to another substrate, may beprovided on one substrate to bond multiple substrates to one substrate.

This smooth surface is formed to surface roughness that enables thesurface roughness of the formed amorphous oxide thin film to be 0.5 nmor less in arithmetic mean height Sa (ISO 4287) when the amorphous oxidethin film described below is formed on this smooth surface. When thesmooth surface is surface-activated and the above-described amorphousoxide thin film is superimposed on it, the smooth surface of thesubstrate itself is formed to an arithmetic mean height Sa of 0.5 nm orless.

It is preferable that the gas adsorption layer, natural oxidation layer,or other altered layers are removed from the smooth surface of thesubstrate before the amorphous oxide thin film is formed. For example,the above-described altered layer can be removed by a known wet processsuch as washing with chemicals, and after the removal of theabove-described altered layer, a substrate that has beenhydrogen-terminated to prevent gas adsorption again can be suitablyused.

The removal of the altered layer is not limited to the wet processdescribed above, but can also be performed by a dry process, and thealtered layer such as the gas adsorption layer and natural oxidationlayer can be removed by reverse sputtering or other means throughbombarding with rare gas ions in a vacuum vessel.

In particular, when the altered layer is removed by a dry process asdescribed above, in order to prevent gas adsorption and oxidation on thesurface of the substrate until the amorphous oxide thin film is formedafter the removal of the altered layer, it is preferable to perform theremoval of the altered layer in the same vacuum as that used to form theamorphous oxide thin film, and to form the amorphous oxide thin filmfollowing the removal of the altered layer, and it is more preferable toremove the altered layer using an ultra-pure inert gas to prevent there-formation of an oxidized layer or the like after the removal of thealtered layer.

The structure that can be bonded to the substrate is not particularlylimited and various structures can be bonded to the substrate, includingsingle crystal, polycrystalline, amorphous, and glassy structures. Whenthe amorphous oxide thin film described below is formed on only one ofthe two substrates and the other substrate is bonded without forming anamorphous oxide thin film, the bonding surface of the other substratewithout the thin film must be either formed with an oxide thin film sothat chemical bonding occurs, or activated by introducing the substrate,whose surface has been hydrophilically treated outside the vacuumvessel, into the vacuum vessel, or by removing the oxidized andcontaminated layers on the substrate surface by dry etching in the samevacuum as that used to form the amorphous oxide thin film, therebyfacilitating the occurrence of chemical bonding.

[Amorphous Oxide Thin Film]

(1) General Materials

The amorphous oxide thin film used for bonding may be made of any oxidethat is stable in a vacuum and in air, and amorphous oxide thin filmsformed of various oxides may be used.

(2) Material Selection Based on Electronegativity or Ionicity

Chemical bonding in oxide thin films is a state of coexistence ofcovalent and ionic bonding. However, in an amorphous oxide thin filmwith high covalent bonding properties, surface atoms stabilize theirenergy states by covalently bonding with each other in a two-dimensionalmanner, making it difficult for chemical bonding to occur at the contactinterface when in contact with other amorphous oxide thin film or thesurface of the substrate.

Therefore, the more an amorphous oxide thin film with high ionic bondingproperties is used for bonding, the higher the bonding performance(bonding strength) becomes. In general, the electronegativity of theelements other than oxygen (oxide-forming elements) that form the oxideof the amorphous oxide thin film is smaller than the electronegativityof oxygen (3.44), and the larger the difference between the two, thegreater the ionic connectivity.

When the electronegativity of the above-described oxide-forming elementsis A and the electronegativity of oxygen is B, the degree of ionicbonding between them (referred to as “ionicity” in the presentinvention) is given by the following equation:

Ionicity (%)=[1−exp{−0.25(B−A)²}]×100

In the chemical bonding of the present invention, it is preferable tobond by forming an amorphous oxide thin film in which the difference(B−A) between the electronegativity B of oxygen and theelectronegativity A of the oxide-forming element is 1.4 or more, or theionicity is 40% or more.

In particular, when high bonding strength is required, bonding ispreferably performed by forming an amorphous oxide thin film in whichthe difference (B−A) between the electronegativity B of oxygen and theelectronegativity A of the oxide-forming element is 1.67 or more, orhaving ionicity of 50% or more.

Examples of the oxide-forming element with a difference (B−A) of 1.67 ormore from the electronegativity B of oxygen, or with an ionicity of 50%or more include Be, Mg, Al, Sc, Ti, V, Cr, Mn, Zn, Y, Zr, Nb, Hf, Ta orthe like.

Examples of the oxide-forming element with a difference (B−A) of 1.4 ormore from the electronegativity B of oxygen, or with an ionicity of 40%or more include, in addition to the oxide-forming elements listed above,Fe, Co, Ni, Cu, Ag, Ge, Ga, In, Sn, B, Si or the like.

In addition, oxides containing alkali metals, alkaline earth metals, andlanthanides, which have extremely high ionic bonding properties, furtherincrease the ionicity, so even better bonding performance can beexpected. Examples of the element include Li, Na, K, Ca, Rh, Sr, Cs, Ba,La, Ce, Pr, Nd, Yb or the like.

The amorphous oxide thin film may be a composition-modulated film inwhich the composition of the forming elements is changed in thethickness or in-plane direction, and in particular, may be a film inwhich only a few atomic layers of the thin film surface arecompositionally modulated to a composition with high ionic bonding.

Alternatively, it may be a multilayer structure with an amorphous oxidethin film with high ionic bonding properties on the surface.

Furthermore, since the spatial atomic positions of amorphous materialsare not as clear as those of crystalline materials, chemical bonding canbe achieved between amorphous oxide thin films of different materials ifchemical bonding can be achieved at the contact interface. Therefore, ina configuration in which an amorphous oxide thin film is formed on eachof the smooth surfaces of one substrate and the other substrate forbonding, the amorphous oxide thin film formed on the smooth surface ofone substrate (first oxide thin film) and the amorphous oxide thin filmformed on the smooth surface of the other substrate (second oxide thinfilm) may be formed by oxides composed of different oxide-formingelements.

In a configuration where an amorphous oxide thin film is formed on onesubstrate only, and the amorphous oxide thin film formed on onesubstrate is superimposed on the smooth surface of the activated othersubstrate for bonding, the material of the substrate may be an oxide ora semiconductor such as Si, as long as the substrate can be activated tomake the surface easy to chemically bond, and the material is notparticularly limited.

As described above, since the surface diffusion coefficient of oxideatoms is very small, the bonding interface of the bonded amorphous oxidethin films can have areas of lower density than the density of theamorphous oxide thin films used for bonding (low-density portions),however there are no application problems because bonding can beachieved even with the occurrence of such low-density portions.

(3) Selection Based on the Physical Properties of the Oxide (Optical,Electromechanical, Etc.)

In the above description, from the viewpoint of bonding performance, theamorphous oxide thin film was selected on the basis of itselectronegativity or ionicity. The preferred oxide-forming elements maybe selected in place of, or in combination with, the above-describedselection based on electronegativity or ionicity, taking into accountthe engineering application aspects (for example, refractive index andelectromechanical coefficient).

For example, an amorphous oxide thin film with an appropriate opticalrefractive index, transmittance, and others is selected for bondingbetween substrates of optical components that transmit light, and forbonding of electronic devices that apply radio waves, ultrasonic waves,or the like is selected, and an amorphous oxide thin film with anappropriate density, electromechanical coefficient, and others isselected for bonding of electronic devices using radio waves, ultrasonicwaves, or the like.

(4) Surface Roughness of Amorphous Oxide Thin Film

In order to achieve strong bonding, the bonding interface between theamorphous oxide thin films (first and second oxide thin films) and theamorphous oxide thin film and the smooth surface of the other substratemust be bonded over a wider area.

However, if the surface of the amorphous oxide thin film is uneven, onlythe contact area between the convex portions will be bonded in a pointcontact state, resulting in a narrow bonding area and low bondingstrength even if bonding is possible.

Furthermore, in oxides, the binding energy of oxygen to metal andsemi-metal elements, which are the oxide-forming elements, is large, sothe transferred atoms are immediately trapped by nearby atoms ofdifferent species. Therefore, the travel distance of the atoms is veryshort, and if only a small gap is generated at the bonding interface,the atoms are trapped by the above-described different atoms on thesurface of the same thin film in this area, making it difficult forchemical bonding to occur at the bonding interface with the differentatoms of the other oxide film (or substrate).

Amorphous oxide thin films, because of their amorphous structure, differfrom thin films with a crystalline structure in that their atoms existrandomly. However, like stable crystalline oxides, their compositionbased on stoichiometry is often stable even in an amorphous structure,and it is still difficult for atoms to migrate.

Therefore, the surface of the amorphous oxide thin film is preferablyable to make contact with the entire area of the film surface at theatomic level during bonding so that atoms can move over a wide area ofthe bonding interface and bond with sufficient strength.

Such atomic-level contact can be achieved by making the surfaceroughness (arithmetic mean height Sa) of the amorphous oxide thin filmas large as that of a unit cell when the oxide constituting theamorphous oxide thin film is crystalline.

Table 2 below shows the crystal structures and lattice constants oftypical oxides.

As is clear from Table 2, the lattice constants of the typical oxideslisted below are from 0.3 to 0.5 nm. In order to make the surfaceroughness of the amorphous oxide thin film as large as the unit cell ofthe oxide, the surface roughness should be 0.5 nm or less, which is theupper limit of the numerical range of the lattice constant, preferablysufficiently smaller than 0.5 nm, and even more preferably 0.3 nm orless, which is the lower limit of the numerical range of the abovelattice constant, thereby making the contact at the bonding interface atthe atomic level.

TABLE 2 Crystal structures and lattice constants of typical oxidesComposition TiO₂ ZrO₂ ZnO MgO Crystal structure Tetragonal TetragonalHexagonal Tetragonal (rutile type) (6 mm) Lattice constant a = 0.459 a =0.515 a = a = 0.421 (nm) c = 0.296 ( b = )0.325 c = 0.521

(5) Film Formation Method

The method for forming an amorphous oxide thin film is not particularlylimited as long as it is a vacuum film forming method capable of formingan oxide thin film having an amorphous structure on a smooth surface ofa substrate in vacuum, and the amorphous oxide thin film may be formedby various known methods.

The amorphous oxide thin film formed by such a vacuum film formingmethod has many structural defects inside the film due to the rapidcooling (quenching) of high-temperature gas-phase and liquid-phase atomsthat reach the smooth surface of the substrate during film formation,which makes it easy for atoms to move, and therefore easy for chemicalbonding to occur at the bonding interface.

In particular, the thin film can incorporate a large amount of oxygendeficiency and supersaturated oxygen, and sputtering, which is easy tocontrol these elements, and vapor deposition in combination with oxygenplasma (oxygen radicals) can be suitably used for the formation of theamorphous oxide thin film in the present invention.

When forming an amorphous oxide thin film by sputtering or vapordeposition in combination with oxygen plasma (oxygen radicals), thestarting material for film formation itself may be an oxide by, forexample, sputtering an oxide target or vapor depositing an oxide solid.Alternatively, an amorphous oxide thin film may be formed on a smoothsurface of a substrate by, for example, depositing the oxide produced byreacting an oxide-forming element with oxygen in a vacuum vessel, orreactive sputtering.

It is also possible to increase the number of defects inside a film bycontrolling oxygen deficiency and supersaturated oxygen to increase theatomic mobility and thereby improve the bonding performance, and it isalso possible to form a film under conditions where only a few atomiclayers of the surface layer of the amorphous oxide thin film are in sucha defect-rich state.

In general, the surface roughness of amorphous oxide thin filmsincreases as the thickness increases. Therefore, when it is necessary toform a relatively thick amorphous oxide thin film, the film may beformed using the energy treatment sputtering (ETS) method, in whichsputtering deposition and ion etching are performed simultaneously, toobtain an amorphous oxide thin film with the above-described surfaceroughness (arithmetic mean height Sa). This ETS method enables theformation of thick amorphous oxide thin films while maintaining smallsurface roughness.

The ETS method also has significant industrial advantages, such as theability to form thick oxide thin films with small surface roughness evenwhen the surface roughness of the substrate is relatively large, and theelimination of the need for high-precision polishing of the substratesurface.

(6) Degree of Vacuum

Impurity gases such as oxygen, water, and carbon remaining in the vacuumvessel are incorporated into the amorphous oxide thin film to be formed,and degrade the physical properties of the oxide thin film.

In addition, when impurity gases such as oxygen, water, and carbon inthe vacuum vessel are adsorbed on the surface of the formed amorphousoxide thin film, they stabilize the chemical state of the surface andinhibit the chemical bonding of the amorphous oxide thin film at thebonding interface.

Therefore, the ultimate vacuum of the vacuum vessel must be better than10⁻³ Pa, which is equal to or less than one hundredth of 10⁻¹ Pa wherethe mean free path of residual gas is equal to the size of the vacuumvessel.

In order to suppress gas adsorption on the surface of the amorphousoxide thin film, ultimate vacuum is more preferably better than 10⁻⁴ Pa,which is equivalent to 1 Langmuir.

It is even better and more ideal to perform thin film deposition andbonding in an ultra-high vacuum environment of 10⁻⁶ Pa or lower, whilemaintaining the purity of the oxygen and other additive gases.

(7) Thickness of Amorphous Oxide Thin Film to be Formed

In order to obtain the physical properties of an amorphous oxide thinfilm, the thickness of the film must be at least equal to or greaterthan the lattice constant (from 0.3 to 0.5 nm from Table 2 above) whenthe oxide constituting the amorphous oxide thin film to be formed iscrystalline, and the lower limit is 0.3 nm, preferably 0.5 nm.

On the other hand, when insulating properties are required for anamorphous oxide thin film, a thick thin film may be required from theviewpoint of breakdown voltage. When optical properties are required foran amorphous oxide thin film, a thin film with a certain thickness maybe required from the viewpoint of wavelength. However, in general filmformation methods, increasing the thickness increases the surfaceroughness, which degrades the bonding performance.

In this regard, according to the ETS method described above, it is alsopossible to form amorphous oxide thin films with small surface roughnesswhile increasing the thickness. However, a very long deposition time isrequired to deposit an amorphous oxide thin film of 5 μm or more, whichmakes it difficult to form industrially. Therefore, the upper limit ofthe thickness of the amorphous oxide thin film is 5 μm, preferably 1 μm.

Therefore, the thickness of the amorphous oxide thin film is preferablyfrom 0.3 nm to 5 μm, and more preferably from 0.5 nm to 1 μm.

(8) Others

In the chemical bonding method of the present invention, bonding canalso be performed by forming an amorphous oxide thin film only on thesmooth surface of one substrate to be bonded, activating the surface ofthe smooth surface of the other substrate to make it easy to chemicallybond, and superimposing the smooth surface of the one substrate on whichthe amorphous oxide thin film is formed.

In such a bonding method, the activation of the smooth surface of theother substrate may be performed by introducing the substrate whosesmooth surface has been hydrophilically treated outside the vacuumvessel into the vacuum vessel, or by removing the oxidized orcontaminated layer on the smooth surface of the other substrate by dryetching or other means in the same vacuum as that used to form theamorphous oxide thin film.

The material of the other substrate may be an oxide or a semiconductorsuch as Si, as long as the substrate can be activated to make thesurface easy to chemically bond, and the material is not particularlylimited.

Thus, by using a bonding method in which the amorphous oxide thin filmis formed only on the smooth surface of one of the substrates, theamorphous oxide thin film can also be used for electrical insulationbetween the substrates to be bonded and for adjusting the opticalproperties between the substrates.

EXAMPLES

The bonding test using the chemical bonding method of the presentinvention is described below.

(1) Test Example 1 (Bonding Using TiO₂ Amorphous Thin Film)

(1-1) Test Outline

As an amorphous oxide thin film, TiO₂ thin film with an amorphousstructure (hereinafter referred to as “TiO₂ thin film”) was formed onthe smooth surface of the substrate, and the change in surface roughnessof the TiO₂ film formed with respect to the change in thickness wasconfirmed.

In addition, the following three substrates (all 2 inches in diameter)were bonded using TiO₂ thin film to check the bonding state and measurethe bonding strength (surface free energy at the bonding interface) γ.

Substrate to be Bonded

Substrate combination 1: Quartz substrate-quartz substrate

Substrate combination 2: Sapphire substrate-sapphire substrate

Substrate combination 3: Si wafer-Si wafer

The electronegativity of titanium (Ti), an oxide-forming element in TiO₂thin film, is 1.54, the difference between the electronegativity ofoxygen (O) (3.44) and that of Ti (1.54) is 1.9, and the ionicity of Tiis 59.4%.

(1-2) Bonding Method

Two substrates were set in a vacuum vessel with an ultimate vacuum equalto or less than 1×10⁻⁶ Pa, and a TiO₂ thin film was formed on the smoothsurface of each of the two substrates by RF magnetron sputtering.

Following the formation of the TiO₂ thin film, the TiO₂ thin filmsformed on the smooth surfaces of each of the two substrates weresuperimposed on each other in the same vacuum as that used to form theTiO₂ thin film, and the substrates were bonded by pressurizing them at apressure of about 1 MPa for 10 seconds without heating.

After bonding, the samples unheated or heat-treated in air at 100° C.,200° C., and 300° C. for 5 minutes were prepared.

Among the above-described substrates, for the bonding of the quartzsubstrate-quartz substrate in the substrate 1, the thickness of the TiO₂thin film formed on the bonding surfaces of both substrates was variedto 2 nm, 5 nm, 10 nm, and 20 nm per side, and the bonded samples wereprepared using the TiO₂ thin films of different thicknesses.

For the bonding of the sapphire substrate-sapphire substrate in thesubstrate 2 and the Si wafer-Si wafer in the substrate 3, the bondingwas performed with a thickness of 5 nm per side.

(1-3) Measurement Method

(1-3-1) Measurement of Surface Roughness

The change in the surface roughness of the TiO₂ thin film (beforebonding) with respect to the change in the thickness of the TiO₂ thinfilm was measured.

The arithmetic mean height Sa (ISO 4287) was measured as the surfaceroughness, and the measurement was performed on a 2 μm square area byatomic force microscope (AFM).

(1-3-2) Measurement of Bonding Strength (Surface Free Energy at BondingInterface) γ

The magnitude of the bonding strength (surface free energy of thebonding interface) γ of the substrates bonded under each of the abovebonding conditions was measured using the “blade method”.

Here, the “blade method” evaluates the bonding strength (surface freeenergy at the bonding interface) γ based on the peeling length L fromthe tip of the blade when the blade is inserted into the bondinginterface of the two substrates, as indicated in FIG. 23, and thebonding strength γ is expressed by the following equation [M. P.Maszara. G. Goetz. A. Cavigila and J. B. McKitterick: J. Appl. Phys. 64(1988) 4943]:

γ=⅜×Et ³γ² /L ⁴

where E is the Young's modulus of the wafer, t is the thickness of thewafer, and γ is ½ the thickness of the blade.

(1-4) Test Results

(1-4-1) Surface Roughness

FIG. 1 shows the change in the surface roughness (arithmetic mean heightSa) of the TiO₂ thin film (before bonding) with respect to the change inthe thickness of the TiO₂ thin film.

The arithmetic mean height Sa was the smallest at 0.18 nm for athickness of 2 nm, and was 0.23 nm for a thickness of 20 nm, although itincreased slightly as the thickness increased.

Thus, the surface roughness of the TiO₂ thin film used in this exampleis 0.5 nm or less in all cases, which is sufficiently smaller than thelattice constant of TiO₂ (a=0.459, c=0.296: see Table 2).

(1-4-2) Bonding Strength (Surface Free Energy of Bonding Interface) γ

FIG. 2 shows the relationship between the magnitude of the bondingstrength (surface free energy at the bonding interface) γ of the quartzsubstrate-quartz substrate bonded using the TiO₂ thin film and thechange in the TiO₂ thin film (2, 5, 10, and 20 nm) used for bonding, foreach heating condition (unheated, 100° C., 200° C., and 300° C.).

The bonding strength (surface free energy at the bonding interface) γafter bonding was from 1.0 to 0.62 J/m² even for the unheated samples,and increased as the heat treatment temperature increased. After theheat treatment at 300° C., the bonding strength γ of the samples bondedusing the TiO₂ thin films of any thickness exceeded 2 J/m², with thehighest bonding strength γ reaching 2.9 J/m² (thickness: 5 nm, heattreatment temperature: 300° C.).

It was also confirmed that the bonding of the sapphiresubstrate-sapphire substrate and the Si wafer-Si wafer bonded using the5 nm thick TiO₂ thin film showed almost the same bonding strength γ asthat of the quartz substrate-quartz substrate bonding.

In the Si wafer-Si wafer bonding, the bonding strength γ was so largethat it could not be evaluated by the blade method after heat treatmentat 300° C. (the blade could not enter the bonding interface, and if itwas inserted forcibly, the Si wafer would break), and it was confirmedthat the bonding strength γ was higher than the breaking strength of theSi wafer.

Therefore, it was confirmed that bonding using the TiO₂ thin film can beperformed with industrially usable strength regardless of the substratematerial, thickness, presence or absence of heat treatment afterbonding, and heat treatment temperature.

(1-4-3) Bonding State

FIG. 3 shows a transmission electron microscope (TEM) image of a samplecross section of a Si wafer-Si wafer bonded with a 5 nm thick (one side)TiO₂ thin film and then heat-treated at 300° C.

The layer that appears white between the Si wafer and the TiO₂ thin filmis a natural oxide layer of Si that exists on the Si wafer surface. Thebonding interface between the TiO₂ films (first and second oxide thinfilms) is bonded without gaps.

It is confirmed that there is a slight bright area at the bondinginterface between the TiO₂ thin films, and that at the bondinginterface, the density of the TiO₂ thin films is slightly reduced(low-density portion) near the bonding interface.

(2) Test Example 2 (Bonding Using 8 Mol % Y₂O₃—ZrO₂ Amorphous Thin Film)

(2-1) Test Outline

As an amorphous oxide thin film, Y₂O₃—ZrO₂ thin film with amorphousstructure containing 8 mol % Y₂O₃ (hereinafter referred to as “Y₂O₃—ZrO₂thin film”) was formed, and the change in surface roughness with respectto the change in thickness was measured.

The bonding of two quartz substrates with a diameter of 2 inches wasperformed using Y₂O₃—ZrO₂ thin film to confirm the bonding state and tomeasure the bonding strength.

Here, Y₂O₃—ZrO₂ is called “stabilized zirconia”. A small amount ofyttria (Y₂O₃) was added as a stabilizer because pure zirconia (ZrO₂) isdifficult to sinter due to the large volume change caused by the phasetransition associated with high temperature changes and cracks in thematerial during cooling.

The electronegativity of zirconium (Zr), an oxide-forming element inY₂O₃—ZrO₂ thin film, is 1.33, the difference between theelectronegativity of oxygen (O) (3.44) and that of Zr (1.33) is 2.11,and the ionicity of Zr is 67.1%.

The electronegativity of yttrium (Y) is 1.22, the difference between theelectronegativity of oxygen (O) (3.44) and that of Y (1.22) is 2.22, andthe ionicity of Y is 70.8%.

(2-2) Bonding Method

Two quartz substrates were set in a vacuum vessel with an ultimatevacuum equal to or less than 1×10⁻⁶ Pa, and Y₂O₃—ZrO₂ thin films wereformed on the bonding surfaces of each of the two quartz substrates byRF magnetron sputtering.

Following the formation of the Y₂O₃—ZrO₂ thin film, the Y₂O₃—ZrO₂ thinfilms formed on the bonding surfaces of the two quartz substrates weresuperimposed on each other in the same vacuum as that used to form theY₂O₃—ZrO₂ thin film, and the bonding was performed by pressurizing thequartz substrates at a pressure of about 1 MPa for 10 seconds withoutheating them.

The thickness of the Y₂O₃—ZrO₂ thin film on the bonding surface of thequartz substrate was varied to 2 nm, 5 nm, 10 nm, and 20 nm per side,and the surface roughness (arithmetic mean height Sa) of Y₂O₃—ZrO₂ thinfilms at each thickness before bonding was measured by atomic forcemicroscopy (AFM) in the same manner as for the above-described TiO₂ thinfilm, and bonding was performed using Y₂O₃—ZrO₂ thin films formed ateach thickness.

In addition, quartz substrates bonded with each of the above thicknesseswere heat-treated in air for 5 minutes without heating or at 100° C.,200° C., and 300° C., respectively, and the bonding strength (surfacefree energy of the bonding interface) γ of each was measured by the“blade method” described above.

(2-3) Test Results

(2-3-1) Surface Roughness Sa and Bonding Strength γ

The measurements of the surface roughness Sa of the Y₂O₃—ZrO₂ thin filmand the bonding strength (surface free energy of the bonding interface)γ of the quartz substrates bonded under each condition are shown inTable 3 below.

TABLE 3 Surface roughness and bonding strength of Y₂O₃—ZrO₂ thin filmThickness (nm) 2 5 10 20 Surface roughness Sa (nm) 0.17 0.19 0.24 0.27Bonding Unheated Unmeasurable 1.43 0.45 0.21 strength γ 100° C.Unmeasurable 1.51 0.53 0.24 (J/m²) 200° C. Unmeasurable 1.81 0.63 0.33300° C. Unmeasurable Unmeasurable 0.76 0.38

The surface roughness Sa increased gradually as the thickness increased,however it was still 0.27 nm even at a thickness of 20 nm, and themaximum value was sufficiently small compared to 0.5 nm, and alsosufficiently small compared to the lattice constant of ZrO₂ (a=0.515 nm:see Table 2), the main component.

In the measurement results of the bonding strength (surface free energyof the bonding interface) γ, “Unmeasurable” in Table 3 indicates thatthe bonding strength was so strong that it was impossible to insert ablade into the bonding interface (the quartz substrate would have brokenif a blade was forcibly inserted).

For the quartz substrates bonded using a 2 nm thick Y₂O₃—ZrO₂ thin film,it has been confirmed that a large bonding strength exceeding thebreaking strength of the quartz substrate, which cannot be measured bythe blade method, has already been obtained in the unheated state.

For the quartz substrates bonded using 5 nm thick Y₂O₃—ZrO₂ thin films,a bonding strength γ of 1.43 J/m² was obtained immediately afterbonding. The bonding strength increased with increasing the heattreatment temperature, and after heat treatment at 300° C., the bondingwas so strong that it could not be evaluated by the blade method.

The bonding strength γ decreased with increasing the thickness of theY₂O₃—ZrO₂ thin film used for bonding, and at a thickness of 20 nm, thebonding strength γ immediately after bonding was about 0.21 J/m², andeven after heat treatment at 300° C., it remained at about 0.38 J/m².

Thus, the decrease in the bonding strength γ with increasing thicknessis mainly due to the increase in surface roughness of the Y₂O₃—ZrO₂ thinfilm with increasing thickness. However, industrially usable bondingstrength γ was obtained even at the maximum thickness, and it wasconfirmed that strong bonding could be achieved for all samples.

When the thicknesses were set to 2 nm and 5 nm, the bonding using theY₂O₃—ZrO₂ thin film achieved a significantly larger bonding strength γthan the bonding using the TiO₂ thin film (Example 1).

On the other hand, when the thickness was set to 10 nm and 20 nm, thebonding strength of the bonding using the TiO₂ thin film was higher thanthat of the bonding using Y₂O₃—ZrO₂ thin film.

These results can be attributed to the higher bonding strength obtainedin the Y₂O₃—ZrO₂ thin film with lower electronegativity (higherionicity) as a result of the surface roughness Sa being almost identicalbetween the Y₂O₃—ZrO₂ and TiO₂ thin films when the thickness was 2 nmand 5 nm.

On the other hand, when the thicknesses was 10 nm and 20 nm, the surfaceroughness Sa of the Y₂O₃—ZrO₂ thin film was larger than that of the TiO₂thin film, which likely resulted in the larger bonding strength γ forthe TiO₂ thin film.

Therefore, it was confirmed that the smaller the electronegativity(higher ionicity) of the oxide-forming elements and the smaller thesurface roughness Sa of the amorphous oxide thin film, the stronger thebonding could be.

(2-3-2) Bonding State

FIGS. 4 and 5 show cross-sectional transmission electron microscopy(TEM) images of Si wafer-Si wafer samples bonded using Y₂O₃—ZrO₂ thinfilm with a thickness of 5 nm (one side).

FIG. 4 shows a sample in an unheated state after bonding, and FIG. 5shows a sample that was heat-treated at 300° C. for 5 minutes in airafter bonding.

In both samples, the layer that appears white between the Si wafer andthe Y₂O₃—ZrO₂ thin film is Si oxide formed on the substrate surface.

In both samples, the bonding interface between Y₂O₃—ZrO₂ thin films isbonded without gaps.

It is confirmed that there is a slight bright area at the bondinginterface between the Y₂O₃—ZrO₂ thin films, and that at the bondinginterface, there is a low-density portion where the density of theY₂O₃—ZrO₂ thin films is slightly reduced near the bonding interface.

In the sample without heat treatment (FIG. 4), a slightlymicrocrystallized area is observed in the Y₂O₃—ZrO₂ thin films.

These microcrystals were formed immediately after the film was formed,however even with the presence of such a small amount of microcrystals,bonding was possible without any problems. Therefore, it was confirmedthat the amorphous oxide thin film to be formed does not necessarilyhave to have a completely amorphous structure, however even if itcontains a small amount of crystalline material in the amorphousstructure, there is no problem in bonding.

In the bonded sample using the Y₂O₃—ZrO₂ thin film, the lattice image ofmicrocrystals in the amorphous material is continuously observed beyondthe bonding interface. From this, it is confirmed that atomic diffusionoccurs at the bonding interface and crystal lattice rearrangement occursin this bonding method, and therefore, the above bonding is accompaniedby atomic diffusion at the bonding interface, and the occurrence of suchatomic diffusion is considered to contribute to a high-strength bondingwith no gaps at the bonding interface.

Since the above-described TiO₂ thin film has a completely amorphousstructure and there are no crystal grains, the occurrence of atomicdiffusion cannot be confirmed by TEM images. However, since the bondinginterface is similarly bonded without gaps and high strength bonding isobtained, it is considered that atomic diffusion at the bondinginterface occurred to a small extent in the bonding using the TiO₂ thinfilm described above and in the bonding using other amorphous oxide thinfilms described below.

Furthermore, in the sample heat-treated at 300° C. after bonding (FIG.5), crystallization progressed inside the Y₂O₃—ZrO₂ thin film due to theheat treatment, and it turned into an almost crystalline thin film.However, chemical bonding can be performed if the material is amorphousin the state before bonding, and crystallization after bonding is not aproblem for bonding.

(3) Test Example 3 (Bonding Using Y₂O₃ Amorphous Thin Film)

(3-1) Test Outline

As an amorphous oxide thin film, an amorphous Y₂O₃ thin film(hereinafter referred to as “Y₂O₃ thin film”) was formed, and the changein surface roughness with respect to the change in thickness wasmeasured.

The bonding of two quartz substrates with a diameter of 2 inches wasperformed using Y₂O₃ thin film to confirm the bonding state and tomeasure the bonding strength.

The electronegativity of yttrium (Y), which is an oxide-forming elementin the Y₂O₃ thin film, is 1.22, the smallest among the materials used inthis application, and the difference between the electronegativity ofoxygen (O) (3.44) and that of Y (1.22) is 2.22, and the ionicity of Y is70.8%.

(3-2) Bonding Method

The Y₂O₃ thin films formed on the bonding surfaces of two quartzsubstrates were superimposed on each other in the same manner as in thecase of bonding using Y₂O₃—ZrO₂ thin films (Test Example 2) describedabove, and the bonding was performed by pressurizing the quartzsubstrates at a pressure of about 1 MPa for 10 seconds without heatingthem.

The thickness of the Y₂O₃ thin film on the bonding surface of the quartzsubstrate was varied to 2 nm, 5 nm, 10 nm, and 20 nm per side, and thesurface roughness (arithmetic mean height Sa) of Y₂O₃ thin films at eachthickness before bonding was measured by atomic force microscopy (AFM),and bonding was performed using Y₂O₃ thin films formed at eachthickness.

In addition, quartz substrates bonded with each of the above thicknesseswere heat-treated in air for 5 minutes without heating or at 100° C.,200° C., and 300° C., respectively, and the bonding strength (surfacefree energy of the bonding interface) γ of each was measured by the“blade method” described above.

(3-3) Test Results

(3-3-1) Surface Roughness Sa and Bonding Strength γ

The measurements of the surface roughness Sa of the Y₂O₃ thin film andthe bonding strength (surface free energy of the bonding interface) γ ofthe quartz substrates bonded under each condition are shown in Table 4below.

TABLE 4 Surface roughness and bonding strength of Y₂O₃ thin filmThickness (nm) 2 5 10 20 Surface roughness Sa (nm) 0.15 0.15 0.15 0.20Bonding Unheated Unmeasurable Unmeasurable 1.8 0.024 strength γ 100° C.Unmeasurable Unmeasurable 1.9 0.39 (J/m²) 200° C. UnmeasurableUnmeasurable 2.0 0.65 300° C. Unmeasurable Unmeasurable 2.3 0.95

The surface roughness Sa increased gradually with increasing thethickness, however it was still 0.20 nm even at a thickness of 20 nm,and the maximum value was small enough to be 0.5 nm.

In the results of the measurement of the bonding strength (surface freeenergy of the bonding interface) γ, for the thicknesses of 2 nm and 5nm, a large bonding strength exceeding the breaking strength of quartzwas already obtained immediately after bonding (without heating).

Even for a thickness of 10 nm, a bonding strength of 1.8 J/m² wasobtained immediately after bonding (unheated), and the bonding strengthγ increased with increasing the heat treatment temperature, reaching 2.3J/m² after heat treatment at 300° C.

For a thickness of 20 nm, the bonding strength γ immediately afterbonding (unheated) was about 0.024 J/m², however increased to 0.95 J/m²after heat treatment at 300° C. The difference in γ with thickness ismainly due to the difference in surface roughness.

Thus, the bonding performance of the Y₂O₃ thin film was superior to thatof the other materials.

(3-3-2) Bonding State

FIG. 6 shows a cross-sectional transmission electron microscopy (TEM)image of a sample of Si wafer-Si wafer bonded with an Y₂O₃ thin filmwith a thickness of 5 nm (one side).

FIG. 6 shows a sample in an unheated state after bonding.

The layer that appears white between the Si wafer and the Y₂O₃ thin filmis a layer of Si oxide formed on the substrate surface.

The bonding interface of the Y₂O₃ thin film disappeared, indicating thatit has excellent bonding performance.

The Y₂O₃ thin film was observed to have very short range lattice fringesin some places, indicating that it was an amorphous layer containingmicrocrystals, and it was confirmed that the amorphous oxide thin filmto be formed does not necessarily have to have a completely amorphousstructure, and even if it contains a small amount of crystallinematerial in the amorphous structure, there is no problem in bonding.

(4) Test Example 4 (Bonding Using Nb₂O₅ Amorphous Thin Film)

(4-1) Test Outline

As an amorphous oxide thin film, Nb₂O₅ thin film with an amorphousstructure (hereinafter referred to as “Nb₂O₅ thin film”) was formed, andthe change in surface roughness with respect to the change in thicknesswas measured.

The bonding of two quartz substrates with a diameter of 2 inches wasperformed using the Nb₂O₅ thin film to confirm the bonding state and tomeasure the bonding strength.

The electronegativity of niobium (Nb), an oxide-forming element in theNb₂O₅ thin film, is 1.6, the difference between the electronegativity ofoxygen (O) (3.44) and that of Nb (1.6) is 1.84, and the ionicity of Nbis 57.1%.

(4-2) Bonding Method

The Nb₂O₅ thin films formed on the bonding surfaces of the two quartzsubstrates were superimposed on each other in the same manner as in thecase of bonding using Y₂O₃—ZrO₂ thin films (Test Example 2) describedabove, and the bonding was performed by pressurizing the quartzsubstrates at a pressure of about 1 MPa for 10 seconds without heatingthem.

The thickness of the Nb₂O₅ thin film formed on the bonding surface ofthe quartz substrate was varied to 2 nm, 5 nm, 10 nm, 20 nm, 30 nm, 50nm, 75 nm, and 100 nm per side, the surface roughness (arithmetic meanheight Sa) of the Nb₂O₅ thin film at each thickness before bonding wasmeasured by atomic force microscopy (AFM), and bonding was performedusing the Nb₂O₅ thin films formed at each thickness.

In addition, quartz substrates bonded with each of the above thicknesseswere heat-treated in air for 5 minutes without heating or at 100° C.,200° C., and 300° C., respectively, and the bonding strength (surfacefree energy of the bonding interface) γ of each was measured by the“blade method” described above.

(4-3) Test Results

(4-3-1) Surface Roughness Sa and Bonding Strength γ

The measurements of the surface roughness Sa of the Nb₂O₅ thin film andthe bonding strength (surface free energy of the bonding interface) γ ofthe quartz substrates bonded under each condition are shown in Table 5below.

TABLE 5 Surface roughness and bonding strength of Nb₂O₅ thin filmThickness (nm) 2 5 10 20 30 50 75 100 Surface roughness Sa 0.17 0.170.18 0.18 0.17 0.17 0.17 0.17 (nm) Bonding Unheated 0.54 0.48 0.41 0.400.39 0.41 0.34 0.34 strength γ 100° C. 0.77 0.69 0.57 0.56 0.50 0.480.46 0.47 (J/m²) 200° C. 0.93 1.00 0.83 0.80 0.79 0.82 0.65 0.72 300° C.1.13 1.37 1.23 1.20 1.18 1.22 0.96 1.07

FIG. 7 shows the relationship between the thickness of the Nb₂O₅ thinfilm and the bonding strength γ, and FIG. 8 shows the relationshipbetween the thickness of the Nb₂O₅ thin film and the surface roughnessSa.

The surface roughness Sa was about 0.17 nm with almost no change in thethickness from 2 nm to 100 nm, and the maximum value was sufficientlysmall at 0.5 nm.

The results of the measurement of the bonding strength γ showed that thevalues ranged from 0.34 to 0.54 J/m² immediately after bonding(unheated), however the values increased with the increase in the heattreatment temperature and reached values exceeding about 1 J/m² afterheat treatment at 300° C., with the highest bonding strength γ being1.37 J/m² (thickness: 5 nm). The change in the bonding strength γ withthickness is small, because the surface roughness Sa does not changemuch with respect to thickness.

Even when Si wafers were bonded using Nb₂O₅ thin film with a thicknessof 5 nm, the same level of bonding strength γ as when the quartzsubstrate was bonded was obtained.

(4-3-2) Bonding State

FIG. 9 shows a cross-sectional TEM photograph of a sample of a Si waferbonded with a 5 nm thick Nb₂O₅ thin film and then heat-treated at 300°C.

The layer that appears white between the Si wafer and the Nb₂O₅ thinfilm is a layer of Si oxide formed on the substrate surface.

There are slightly brighter areas at the bonding interface compared tothe interior of the Nb₂O₅ film, indicating that the density of thebonding interface is slightly lower than that of the interior of thethin film, however the bonding interface of the Nb₂O₅ thin film isbonded without gaps.

(5) Test Example 5 (Bonding Using Al₂O₃ Amorphous Thin Film)

(5-1) Test Outline

As an amorphous oxide thin film, an Al₂O₃ thin film with an amorphousstructure (hereinafter referred to as “Al₂O₃ thin film”) was formed, andthe change in surface roughness with respect to the change in thicknesswas measured.

The bonding of two quartz substrates with a diameter of 2 inches wasperformed using the Al₂O₃ thin film to confirm the bonding state and tomeasure the bonding strength.

The electronegativity of aluminum (Al), an oxide-forming element in theAl₂O₃ thin film, is 1.61, the difference between the electronegativityof oxygen (O) (3.44) and that of Al (1.61) is 1.83, and the ionicity ofAl is 56.7%.

(5-2) Bonding Method

The Al₂O₃ thin films formed on the bonding surfaces of the two quartzsubstrates were superimposed on each other in the same manner as in thecase of bonding using Y₂O₃—ZrO₂ thin films (Test Example 2) describedabove, and the bonding was performed by pressurizing the quartzsubstrates at a pressure of about 1 MPa for 10 seconds without heatingthem.

The thickness of the Al₂O₃ thin film on the bonding surface of thequartz substrate was varied to 1 nm, 2 nm, 5 nm, and 10 nm per side, andthe surface roughness (arithmetic mean height Sa) of the Al₂O₃ thinfilms at each thickness before bonding was measured by atomic forcemicroscopy (AFM), and bonding was performed using the Al₂O₃ thin filmsformed at each thickness.

In addition, quartz substrates bonded with each of the above thicknesseswere prepared and heat-treated in air without heating or at 100° C.,200° C., and 300° C. for 5 minutes, and the bonding strength (surfacefree energy of the bonding interface) γ of each was measured by the“blade method” described above.

(5-3) Test Results

(5-3-1) Surface Roughness Sa and Bonding Strength γ

The measurements of the surface roughness Sa of the Al₂O₃ thin film andthe bonding strength (surface free energy of the bonding interface) γ ofthe quartz substrates bonded under each condition are shown in Table 6below.

TABLE 6 Surface roughness and bonding strength of Al₂O₃ thin filmThickness (nm) 1 2 5 10 Surface roughness Sa (nm) 0.15 0.15 0.15 (0.15)0.17 Bonding Unheated 0.50 0.47 0.43 (0.67) 0.34 strength γ 100° C. 0.590.66 0.56 0.47 (J/m²) 200° C. 0.86 0.97 0.80 0.66 300° C. 1.05 1.48 1.19(2.45) 1.07 * In Table 6, the values in parentheses are the measurementsusing Al₂O₃ thin film formed on Si wafer.

FIG. 10 and FIG. 11 show the relationship between the thickness of theAl₂O₃ thin film and the bonding strength γ and the relationship betweenthe thickness of the Al₂O₃ thin film and the surface roughness Sa,respectively.

The surface roughness Sa increased slightly with increasing thethickness, however it was still 0.17 nm at a thickness of 10 nm, and themaximum value was sufficiently small at 0.5 nm.

The results of the measurement of the bonding strength γ showed that thevalues ranged from 0.34 to 0.50 J/m² immediately after bonding(unheated), however the values increased with the increase in the heattreatment temperature and reached values exceeding about 1 J/m² afterheat treatment at 300° C., with the highest bonding strength γ being1.48 J/m² (thickness: 2 nm).

As shown in the results for a thickness of 5 nm, the bonding strength γwas higher for Si wafers bonded than for quartz substrates bonded bothwithout heat treatment and heated at 300° C. In the case of Si waferbonding, the strength reached 2.45 J/m² after heat treatment at 300° C.

(5-3-2) Bonding State

FIG. 12 shows a cross-sectional TEM photograph of a sample of a Si waferbonded using the Al₂O₃ thin film with a thickness of 5 nm, and thenheat-treated at 300° C.

The layer that appears white between the Si wafer and the Al₂O₃ thinfilm is a layer of Si oxide formed on the substrate surface.

The bonding interface of the Al₂O₃ thin film appears brighter overallthan the interior of the thin film, indicating that the density of thebonding interface is slightly lower than that of the interior of thethin film, however the bonding interface of the Al₂O₃ thin film isbonded without gaps.

(6) Test Example 6 (Bonding Using 9.7 wt % SnO₂—In₂O₃ Amorphous ThinFilm)

(6-1) Test Outline

Amorphous oxide thin films of 9.7 wt % SnO₂—In₂O₃ (hereinafterabbreviated as “ITO” with the initial letters of Indium Tin Oxide) withamorphous structure were formed and the change in surface roughness withrespect to the change in the thickness of the formed films was measured.

The bonding of two quartz substrates with a diameter of 2 inches wasperformed using the ITO thin film to confirm the bonding state and tomeasure the bonding strength.

The electronegativity of indium (In) and tin (Sn), which are theoxide-forming elements of the ITO thin film, are 1.78 and 1.96,respectively, and from the composition ratio of SnO₂ and In₂O₃, theelectronegativity of the oxide-forming elements of the ITO thin film canbe considered to be 1.81. The difference between the electronegativityof oxygen (O) (3.44) and that of the oxide-forming elements of the ITOthin film (1.81) is 1.63, and the ionicity of the oxide-forming elementsis 48.5%.

The electronegativity of indium (In) is 1.78, the difference between theelectronegativity of oxygen (O) (3.44) and that of In (1.78) is 1.66,and the ionicity of In is 49.8%.

(6-2) Bonding Method

The ITO thin films formed on the bonding surfaces of the two quartzsubstrates were superimposed on each other in the same manner as in thecase of bonding using Y₂O₃—ZrO₂ thin films (Test Example 2) describedabove, and the bonding was performed by pressurizing the quartzsubstrates at a pressure of about 1 MPa for 10 seconds without heatingthem.

The thickness of the ITO thin film on the bonding surface of the quartzsubstrate was varied to 2 nm, 5 nm, and 10 nm per side, and the surfaceroughness (arithmetic mean height Sa) of the ITO thin films at eachthickness before bonding was measured by atomic force microscopy (AFM).

In addition, quartz substrates bonded with the ITO thin films of 1 nm, 2nm, 5 nm, 10 nm, and 20 nm in thickness per side, respectively, wereprepared and heat-treated in air without heating or at 100° C., 200° C.,and 300° C. for 5 minutes each, and the bonding strength (surface freeenergy of the bonding interface) γ of each was measured by the “blademethod” described above.

(6-3) Test Results

(6-3-1) Surface Roughness Sa and Bonding Strength γ

The results of measuring the surface roughness Sa of the ITO thin filmand the bonding strength (surface free energy of the bonding interface)γ of the quartz substrates bonded under each condition are shown inTable 7 below.

TABLE 7 Surface roughness and bonding strength of ITO thin filmThickness (nm) 1 2 5 10 20 Surface roughness Sa (nm) 0.15 0.15 0.16Bonding Unheated 0.68 0.74 0.52 0.47 0.43 strength γ 100° C. 0.82 0.900.74 0.62 0.62 (J/m²) 200° C. 1.00 1.11 1.00 0.74 1.11 300° C. 1.23 1.381.73 1.11 1.11

FIGS. 13 and 14 show the relationship between the thickness of the ITOfilm and the bonding strength γ and the relationship between thethickness of the ITO film and the surface roughness Sa, respectively.

The surface roughness Sa increased slightly with increasing thethickness, however it was still 0.16 nm at a thickness of 10 nm, and themaximum value was sufficiently small at 0.5 nm.

The results of the measurement of the bonding strength γ showed that thevalues ranged from 0.43 to 0.74 J/m² immediately after bonding(unheated), however the values increased with the increase in the heattreatment temperature and reached values exceeding 1 J/m² after heattreatment at 300° C., with the highest bonding strength γ being 1.73J/m² (thickness: 5 nm).

As a result of bonding sapphire substrates and Si wafers using the ITOthin film with a thickness of 5 nm (one side), it was confirmed that thebonding strength γ was approximately the same as when quartz substrateswere bonded together.

(6-3-2) Bonding State

FIG. 15 shows a cross-sectional TEM photograph of a sample of a Si waferbonded using an ITO thin film with a thickness of 5 nm, and thenheat-treated at 300° C.

The layer that appears white between the Si wafer and the ITO thin filmis a layer of Si oxide formed on the substrate surface.

The bonding interface of the ITO thin film appears brighter overall thanthe interior of the ITO thin film, indicating that the density of thebonding interface is slightly lower than that of the interior of thethin film, however the bonding interface of the ITO thin film is bondedwithout gaps.

The ITO thin film showed very short range lattice fringes in some placesand was an amorphous layer containing microcrystals.

(7) Test Example 7 (Bonding Using Ga₂O₃ Amorphous Thin Film)

(7-1) Test Outline

As an amorphous oxide thin film, the Ga₂O₃ thin film with an amorphousstructure (hereinafter referred to as “Ga₂O₃ thin film”) was formed, andthe change in surface roughness with respect to the change in thicknesswas measured.

The bonding of two quartz substrates with a diameter of 2 inches wasperformed using the Ga₂O₃ thin film to confirm the bonding state and tomeasure the bonding strength.

The electronegativity of gallium (Ga), an oxide-forming element in theGa₂O₃ thin film, is 1.81, the difference between the electronegativityof oxygen (O) (3.44) and that of Ga (1.81) is 1.63, and the ionicity ofGa is 48.5%.

(7-2) Bonding Method

The Ga₂O₃ thin films formed on the bonding surfaces of the two quartzsubstrates were superimposed on each other in the same manner as in thecase of bonding using Y₂O₃—ZrO₂ thin films (Test Example 2) describedabove, and the bonding was performed by pressurizing the quartzsubstrates at a pressure of about 1 MPa for 10 seconds without heatingthem.

The thickness of the Ga₂O₃ thin film on the bonding surface of thequartz substrate was varied to 1 nm, 2 nm, and 5 nm per side, and thesurface roughness (arithmetic mean height Sa) of the Ga₂O₃ thin films ateach thickness before bonding was measured by atomic force microscopy(AFM), and bonding was performed using the Ga₂O₃ thin films formed ateach thickness.

In addition, quartz substrates bonded using the Ga₂O₃ thin films of eachof the above-described thicknesses were heat-treated in air for 5minutes without heating or at 100° C., 200° C., and 300° C.,respectively, and the bonding strength (surface free energy of thebonding interface) γ of each was measured using the “blade method”described above.

(7-3) Test Results

(7-3-1) Surface Roughness Sa and Bonding Strength γ

The results of measuring the surface roughness Sa of the Ga₂O₃ thin filmand the bonding strength (surface free energy of the bonding interface)γ of the quartz substrates bonded under each condition are shown inTable 8 below.

TABLE 8 Surface roughness and bonding strength of Ga₂O₃ thin filmThickness (nm) 1 2 5 Surface roughness Sa (nm) 0.16 0.16 0.18 BondingUnheated 0.82 0.83 0.18 (0.40) strength γ 100° C. 1.00 1.23 0.37 (J/m²)200° C. 1.53 1.53 1.27 300° C. 2.04 1.93 2.22 (2.77) * In Table 8, thevalues in parentheses are the measurements using Ga₂O₃ thin film formedon Si wafer.

FIG. 16 shows the relationship between the thickness of the Ga₂O₃ filmand the bonding strength γ, and FIG. 17 shows the relationship betweenthe thickness of the Ga₂O₃ film and the surface roughness Sa.

The surface roughness Sa increased slightly with increasing thethickness, however it was 0.18 nm at a thickness of 5 nm, which wassufficiently small compared to 0.5 nm.

The results of the measurement of the bonding strength γ showed that thevalues ranged from 0.82 to 0.18 J/m² immediately after bonding(unheated), however with increasing the heat treatment temperature, thebonding strength γ increased, reaching about 2 J/m² after the heattreatment at 300° C., with the highest bonding strength γ being 2.22J/m² (thickness: 5 nm).

The results for the thickness of 5 nm show that the bonding strength γbetween the Si wafers was higher than the bonding strength γ between thequartz substrates, both for the film without heat treatment and for thefilm heated to 300° C. In the case of the Si wafer bonding, the bondingstrength reached 2.77 J/m² after heat treatment at 300° C.

(7-3-2) Bonding State

FIG. 18 shows a cross-sectional TEM photograph of a sample of a Si waferbonded using a Ga₂O₃ thin film with a thickness of 5 nm, and thenheat-treated at 300° C.

The layer that appears white between the Si wafer and the Ga₂O₃ thinfilm is a layer of Si oxide formed on the substrate surface.

The bonding interface of the Ga₂O₃ thin film appears brighter overallthan the interior of the thin film, indicating that the density of thebonding interface is slightly lower than that of the interior of thethin film, however the bonding interface of the Ga₂O₃ thin film isbonded without gaps.

(8) Test Example 8 (Bonding Using GeO₂ Amorphous Thin Film)

(8-1) Test Outline

As an amorphous oxide thin film, a GeO₂ thin film with an amorphousstructure (hereinafter referred to as “GeO₂ thin film”) was formed. Thebonding of two quartz substrates with a diameter of 2 inches wasperformed, the bonding state was confirmed, and the change in bondingstrength γ with respect to the change in the thickness of the formedfilm was measured by the “blade method” described above.

The electronegativity of germanium (Ge), an oxide-forming element in theGeO₂ thin film, is 2.01, the difference between the electronegativity ofoxygen (O) (3.44) and that of Ge (2.01) is 1.43, and the ionicity of Geis 40%.

(8-2) Bonding Method

The GeO₂ thin films formed on the bonding surfaces of the two quartzsubstrates were superimposed on each other in the same manner as in thecase of bonding using Y₂O₃—ZrO₂ thin films (Test Example 2) describedabove, and the bonding was performed by pressurizing the quartzsubstrates at a pressure of about 1 MPa for 10 seconds without heatingthem.

The thickness of the GeO₂ thin film on the bonding surface of the quartzsubstrate was varied to 1 nm, 2 nm, 3 nm, and 5 nm per side, and thesurface roughness (arithmetic mean height Sa) of the GeO₂ thin films ateach thickness before bonding was measured by atomic force microscopy(AFM), and bonding was performed using the GeO₂ thin films formed ateach thickness.

In addition, quartz substrates bonded using the GeO₂ thin films of eachof the above-described thicknesses were heat-treated in air for 5minutes without heating or at 100° C., 200° C., and 300° C.,respectively, and the bonding strength (surface free energy of thebonding interface) γ of each was measured using the “blade method”described above.

The surface roughness Sa of the GeO₂ thin film cannot be measuredbecause the surface reacts with moisture in the air and agglomerates, sothat the surface roughness Sa was not measured in this test example.

(8-3) Test Results

(8-3-1) Surface Roughness Sa and Bonding Strength γ

The results of the measurement of the bonding strength (surface freeenergy of the bonding interface) γ of the quartz substrate with respectto the change in the thickness of the GeO₂ thin film are shown in Table9 below.

TABLE 9 Surface roughness and bonding strength of GaO₂ thin filmThickness (nm) 1 2 3 5 Bonding Unheated 0.08 0.13 0.14 0.09 strength γ100° C. 0.57 0.57 0.58 0.35 (J/m²) 200° C. 0.99 0.99 1.02 0.81 300° C.1.09 1.21 1.16 (1.47) 0.90 * In Table 9, the values in parentheses arethe measurements using GaO₂ thin film formed on Si wafer.

FIG. 19 shows the relationship between the thickness of the GeO₂ thinfilm and the bonding strength γ.

Immediately after bonding (unheated), the bonding strength γ ranged from0.08 to 0.14 J/m², and increased with increasing the heat treatmenttemperature. After heat treatment at 300° C., the bonding strengthexceeded 1 J/m² in the range of from 1 to 3 nm thickness, however eventhe highest bonding strength γ was only about 1.21 J/m² (2 nmthickness).

However, the quartz substrates were bonded to each other regardless ofthe thickness.

As a result of bonding Si wafers using the GeO₂ thin film with athickness of 3 nm (one side), the bonding strength was 1.47 J/m² afterheat treatment at 300° C., which was slightly higher than that ofbonding quartz substrates, however Si wafers were still bonded in thisconfiguration.

Germanium, the oxide-forming element of the GeO₂ thin film, has thehighest electronegativity of 2.01 (the difference from theelectronegativity of oxygen is the smallest at 1.43) and the lowestionicity of 40.0% among the oxide-forming elements of amorphous oxidethin films used in the experiments. Therefore, from the examples ofsuccessful bonding using the GeO₂ thin film, it is confirmed thatbonding can be performed using an amorphous oxide thin film containingan oxide-forming element with a difference of about 1.4 inelectronegativity from oxygen and ionicity of about 40%.

(8-3-2) Bonding State

FIG. 20 shows a cross-sectional TEM photograph of a sample of a Si waferbonded using a GeO₂ thin film with a thickness of 2 nm, and thenheat-treated at 300° C.

The layer that appears white between the Si wafer and the GeO₂ thin filmis a layer of Si oxide formed on the substrate surface.

In the sample after heating at 300° C., the bonding interface betweenthe GeO₂ thin films disappeared, indicating that strong bonding wasobtained. This bonding state was also consistent with the fact that thesamples bonded using the GeO₂ thin film showed a significant increase inbonding strength of 9.3 times when heated to 300° C. compared to theunheated samples. This is because the melting point of the GeO₂ thinfilm is 1115° C., which is lower (compared to other oxides), and theeffect of the heat treatment at 300° C. to promote atomic diffusion atthe bonding interface is relatively greater than that of other oxidethin films.

(9) Test Example 9 (Comparison of Types of Amorphous Oxide Thin Filmsand Bonding Strength)

(9-1) Test Outline

As an amorphous oxide thin film, in addition to the amorphous oxide thinfilms used in Test Examples 1 to 8, SiO₂ thin films with an amorphousstructure (hereinafter referred to as “SiO₂ thin films”) were alsoformed, and the surface roughness Sa of each film was measured andcompared in the same manner as in Test Examples 1 and 2.

The bonding strength γ of quartz substrate-quartz substrates (2 inchesin diameter, neither of which was heated during bonding) bonded usingeach thin film was measured and compared using the “blade method”described above, both when unheated and after bonding and heat treatmentat 300° C. for 5 minutes in air.

(9-2) Test Results

(9-2-1) Measurement Results

The surface roughness Sa of each thin film and the bonding strength γ ofthe quartz substrate-quartz substrate bonded using each thin film areshown in Table 10 below.

TABLE 10 Surface roughness and bonding strength of various amorphousoxide thin films Material Test Electronegativity Surface Bondingstrength γ (J/m²) Example (difference Ionicity Thickness roughnessHeated at No.) from oxygen) (%) (nm) Sa (nm) Unheated 300° C. Y₂O₃ 1.22(2.22) 70.8 5 0.15 Unmeasurable Unmeasurable (Test Example 3) Y₂O₃—ZrO₂1.32* (2.12*) 66.8* 5 0.19 1.43 Unmeasurable (Test Example 2) TiO₂ (Test1.54 (1/9)  59.4 5 0.18 0.77 2.9 Example 1) Nb₂O₅ 1.60 (1.84) 57.1 50.17 0.48 1.37 (Test Example 4) Al₂O₃ 1.61 (1.83) 56.7 2 0.15 0.47 1.48(Test Example 5) ITO (Test 1.81* (1.63)* 48.5* 5 0.15 0.52 1.73 Example6) Ga₂O₃ 1.81 (1.63) 48.5 2 0.16 0.83 1/93 (Test Example 7) SiO₂  1.9(1.54) 44.7 2 0.15 0.045 0.045 GeO₂ 2.01 (1.43) 40.0 2 — 0.13 1.21 (TestExample 8) *The value for Y₂O₃—ZrO₂ was calculated as the weightedaverage of ZrO₂ and Y₂O₃, and the value for ITO was calculated as theweighted average of SnO₂ and In₂O₃.

The surface roughness Sa of the amorphous oxide thin films of bothmaterials was small, less than 0.2 nm.

It has been confirmed that the smaller the electronegativity of theoxide-forming elements in the amorphous oxide thin film, the greater thebonding strength γ, both for the unheated and heated samples.

Comparing the unheated samples after bonding, it can be seen that thebonding strength γ tends to increase with decreasing theelectronegativity of the oxide-forming elements in the amorphous oxidethin film (with increasing the difference between the electronegativityof the oxide-forming elements and that of oxygen).

In particular, for the Y₂O₃ thin film with the smallestelectronegativity (the difference with the electronegativity of oxygenis the largest), the bonding strength γ is so large that it cannot beevaluated by the blade method even in the unheated state (exceeding thefracture strength of the quartz substrate).

With the exception of the SiO₂ thin film, the bonding strength γ of allthe samples increased with heat treatment, and in the case of the sampleusing the Y₂O₃—ZrO₂ thin film, heat treatment at 300° C. increased thebonding strength to a level that could not be evaluated by the blademethod.

The bonding strength γ of the samples bonded using the SiO₂ thin filmwas lower than that of the samples bonded using other amorphous oxidethin films, however the bonding was still possible.

(9-2-2) Relationship Between Electronegativity, Ionicity, and BondingStrength γ

FIGS. 21A and 21B shows the relationship between the electronegativityof the oxide-forming elements of the amorphous oxide thin films used forbonding and the bonding strength γ.

FIGS. 21A and 21B show the relationship between the electronegativityand the bonding strength γ of the samples after bonding at roomtemperature and unheated, and after bonding at room temperature and heattreatment at 300° C. for 5 minutes, respectively.

FIGS. 21A and 21B both show the values of the bonding strength γ whenquartz wafers are bonded using an amorphous oxide thin film with athickness of 2 nm (one side).

As can be seen from FIGS. 21A and 21B, the bonding strength γ increasedwith decreasing the electronegativity (the difference inelectronegativity from oxygen increased), and in particular, the bondingstrength of Y₂O₃ and Y₂O₃—ZrO₂, which have the smallestelectronegativity among the examples (the difference inelectronegativity from oxygen is large), both exceeded the breakingstrength of quartz at a thickness of 2 nm, both when unheated and whenheated at 300° C., and the bonding strength γ could not be measured(however, at a thickness of 5 nm, the bonding strength of Y₂O₃—ZrO₂could not be measured only when heated to 300° C.: see Table 10).

Of these, bonding using Y₂O₃ thin film, which has the lowestelectronegativity (the largest difference from the electronegativity ofoxygen), has excellent bonding performance, as the bonding interfacedisappears even immediately after bonding (unheated), as described withreference to FIG. 6.

On the other hand, although the bonding strength γ decreased withincreasing the electronegativity (the difference in electronegativitywith oxygen decreased), the GeO₂ films with an electronegativity of 2.01(the difference in electronegativity with oxygen is about 1.43), whichcorresponds to an ionic crystallinity of 40%, could be bonded eitherunheated or heated to 300° C. after bonding. This indicates that bondingis possible as long as the electronegativity is 2 or less (thedifference in electronegativity with oxygen is about 1.4 or more).

FIG. 21B shows that the bonding strength γ increases with heat treatmentat 300° C. The tendency for the bonding strength γ to decrease as theelectronegativity increases is similar to the unheated case shown inFIG. 21A.

For materials with large electronegativity, such as GeO₂ and ITO,heating at 300° C. significantly increased the bonding strength by 9.3times for GeO₂ and 3.3 times for ITO compared to the unheated condition.Especially for GeO₂, it has been confirmed that the bonding wasaccompanied by atomic diffusion to the extent that the bonding interfacedisappeared, as described with reference to FIG. 20.

The reason for the increase in the bonding strength γ of the materialwith large electronegativity by the heat treatment at 300° C. is thoughtto be that the melting point tends to be lower for the material withlarge electronegativity.

This is because the lower the melting point (i.e., the higher theelectronegativity), the greater the effect of promoting atomic diffusionat the bonding interface, and the greater the rate of increase inbonding strength, even when the same heat treatment at 300° C. isapplied.

As an example, the melting points of GeO₂ and ITO, for which the rate ofincrease in the bonding strength γ by heat treatment was large, were1115° C. for GeO₂ and about 900° C. for ITO, respectively, which is lessthan half of the melting point of Y₂O₃ (2425° C.), which has thesmallest electronegativity among the examples.

FIGS. 22A and 22B shows the relationship between the ionicity of theoxide-forming elements in the amorphous oxide thin films used forbonding and the bonding strength γ.

FIG. 22A and FIG. 22B show the relationship between the ionicity andbonding strength γ of the samples after bonding at room temperature andunheated, and after bonding at room temperature and heat treatment at300° C. for 5 minutes, respectively.

FIGS. 22A and 22B both show the values of bonding strength γ when quartzwafers are bonded using an amorphous oxide thin film with a thickness of2 nm (one side).

As indicated in FIGS. 22A and 22B, the bonding strength γ increased asthe ionicity increased, and in particular, the bonding strength of Y₂O₃and Y₂O₃—ZrO₂, which have the largest ionicity among the examples,exceeded the breaking strength of quartz in both unheated and heating at300° C. at a thickness of 2 nm, and the bonding strength γ could not bemeasured (however, at a thickness of 5 nm, the Y₂O₃—ZrO₂ bondingstrength could not be measured only at heating at 300° C.: see Table10).

Of these, the bonding using Y₂O₃ thin film, which exhibited the maximumionicity of 70.8%, has excellent bonding performance, as the bondinginterface disappears even immediately after bonding (unheated), asexplained in FIG. 6.

On the other hand, although the bonding strength γ decreases withdecreasing the ionicity, it was confirmed from the bonding example usingthe GeO₂ thin film (Test Example 8) that bonding is possible up to anionicity of about 40%.

FIG. 22B shows that the bonding strength γ increases with heat treatmentat 300° C. The tendency for the bonding strength γ to decrease withdecreasing ionicity is similar to the unheated case shown in FIG. 22A.

For materials with small ionicity, such as GeO₂ and ITO, heating at 300°C. resulted in a significant increase in the bonding strength γ comparedto the unheated case. This significant increase in the bonding strengthγ is thought to be due to the fact that the melting point tends to belower for materials with smaller ionic properties, and even when thesame heat treatment at 300° C. is applied, the lower the melting point(i.e., the smaller the ionic properties), the greater the effect ofpromoting atomic diffusion at the bonding interface, and the greater therate of increase in the bonding strength.

The bonding strength γ of the bonding using Ga₂O₃ thin film was about 14times higher when unheated and about 50 times higher when heat-treatedat 300° C. than that of the bonding using the SiO₂ thin film, and about6.4 times higher when unheated and 1.6 times higher when heat-treated at300° C. than that of the bonding using the GeO₂ thin film. These resultsindicate that a significant increase in the bonding strength γ isobtained when the electronegativity is lower than the electronegativityof Ga, 1.81 (the difference in electronegativity with oxygen is higherthan 1.63), or when the ionicity is higher than 48.5% (≈; 50%).

What is claimed is:
 1. A bonded structure comprising: a first substrate;a second substrate placed opposite to the first substrate; anintermediate layer provided between the first substrate and the secondsubstrate and including a first oxide thin film layered on the firstsubstrate and a second oxide thin film layered on the second substrate;at least one of the first oxide thin film and the second oxide thin filmof the intermediate layer being formed of oxide thin films havingincreased defects; an interface between the first oxide thin film andthe second oxide thin film=being bonded by chemical bonding, and theinterface comprising a low-density portion whose density is lower thanthat of the two oxide thin films.
 2. A bonded structure comprising: afirst substrate; a second substrate placed opposite to the firstsubstrate; an intermediate layer provided between the first substrateand the second substrate and including an oxide thin film havingincreased defects layered on the first substrate; an interface betweenthe oxide thin film of the intermediate layer and the second substratebeing bonded by chemical bonding, and the oxide thin film at the bondedportion having a low-density portion whose density is lower than that ofthe oxide thin film.
 3. The bonded structure according to claim 1,wherein the interface between the first oxide thin film and the secondoxide thin film of the intermediate layer is bonded by chemical bondingwith atomic diffusion.
 4. The bonded structure according to claim 2,wherein the interface between the oxide thin film of the intermediatelayer and the second substrate is bonded by chemical bonding with atomicdiffusion.
 5. The bonded structure according to claim 1, wherein amaterial constituting the oxide thin film of the intermediate layer isdifferent from a material constituting the first substrate or the secondsubstrate.
 6. The bonded structure according to claim 2, wherein amaterial constituting the oxide thin film of the intermediate layer isdifferent from a material constituting the first substrate or the secondsubstrate.
 7. The bonded structure according to claim 3, wherein amaterial constituting the oxide thin film of the intermediate layer isdifferent from a material constituting the first substrate or the secondsubstrate.
 8. The bonded structure according to claim 4, wherein amaterial constituting the oxide thin film of the intermediate layer isdifferent from a material constituting the first substrate or the secondsubstrate.
 9. The bonded structure according to claim 1, wherein both ofthe first oxide thin film and the second oxide thin film of theintermediate layer being formed of oxide thin films having increaseddefects.
 10. The bonded structure according to claim 3, wherein both ofthe first oxide thin film and the second oxide thin film of theintermediate layer being formed of oxide thin films having increaseddefects.
 11. The bonded structure according to claim 5, wherein both ofthe first oxide thin film and the second oxide thin film of theintermediate layer being formed of oxide thin films having increaseddefects.
 12. The bonded structure according to claim 7, wherein both ofthe first oxide thin film and the second oxide thin film of theintermediate layer being formed of oxide thin films having increaseddefects.